Xerographic exposure systems are important for printing. In xerographic printing, lasers or LEDS may be used to expose tiny dots on a photoreceptor surface. The photoreceptor has the property of holding an electrical charge in the absence of light. Illumination of a spot on the photoreceptor by a laser or LED causes the loss of charge at the exposed spot. In a typical xerographic system, charge left on the photoreceptor attracts toner that is then transferred to paper which has a greater charge than the photoreceptor.
Desirable features for xerographic exposure printing systems include printing at high speed, wide format printing, high resolution addressability, elimination of moving mechanical parts, low power consumption, and low profile at the photoreceptor. The first three features are important for achieving performance comparable to offset lithography and occupy a parameter space that lies beyond the speed and width capabilities of polygon raster output scanning (ROS) print engines. Polygon ROS printers typically consist of a laser light source, a modulator, a polygon scanning beam deflector, an optical system of lenses and mirrors, a xerographic marking engine and the electronics to control the printer operation.
The remaining three features are desirable for any xerographic exposure system. Prior art devices that incorporate these features use LED (light emitting diode) or VCSEL (vertical cavity surface emitting laser) print bars requiring a large number of devices and drivers. The present invention allows a solid state laser scanning architecture which simultaneously enables high print speed (70 inches/sec or higher), high resolution (1200 spi or higher), low power (less than 1 watt), requires a very small width on the photoreceptor in the process direction (on the order of a millimeter or less), and is extendible to wide format printing by utilizing stitching and electronic tolerance correction. By multiplexing microscanners with laser sources the overall number of devices and drivers is reduced by more than an order of magnitude compared to LED print bars or VCSEL print bars. The reduction of the number of devices improves the overall prospects for yield and reliability since the number and the difficulty of contacts for the assembly is reduced.
In accordance with this invention, a linear array of lasers faces and is positioned parallel to a linear array of mirrors with the laser array tilted at an angle, .phi., with respect to the mirror array. The tilt angle .phi. is at least large enough so that the laser beams reflected from the mirror are not blocked by the laser array. This means that the tilt angle, .phi., must be larger than the laser divergence angle. Each laser in the array is individually driven by a waveform derived from the photoreceptor image data. The mirrors can each be deflected about an axis bisecting the mirror plane perpendicular to the array axis. The deflection, .theta., of each mirror is individually controlled by a sweep generator waveform. Each laser and corresponding mirror are separated by a distance, d, that is chosen to be short enough so that the laser beam directed at the mirror does not diverge onto adjacent mirrors.
In operation, each mirror scans a number of pixels on the photoreceptor. In other words, the pixels are multiplexed from a single mirror. This multiplexing allows for the reduction of the number of lasers needed to achieve the desired performance characteristics. The number of pixels multiplexed from a single mirror is determined by the mirror pitch, the magnification of the optics, and the print resolution. Additionally, the number of pixels addressable by a mirror depends on the dynamic range of the mirror. The dynamic range of the mirror is the number of individual angular locations resolvable by the scanning mirror; i.e. 2.theta..sub.max /.DELTA..theta. where .DELTA..theta. is the mirror positional accuracy.
Thus, the present invention and its embodiments provide numerous advantages including, but not limited to high print speed, high resolution, low power, low profile at the photoreceptor, and extendibility to wide format printing while reducing the overall number of devices and drivers needed as will be described in further detail below.